It seems that there is a bug with \textwidth and \linewidth inside mixed
columns. Minimum working example:

% interface=en output=pdftex
\definemixedcolumns[StyleColumns][
  n=2,
  separator=rule,
  align=tolerant,
  before={\blank[big]},]
\setupexternalfigures[
  order={svg,pdf,png,jpg},
  location=global,
  %wfactor=fit,]
\setuphead[section][
  after={\startStyleColumns},
  aftersection={\stopStyleColumns},]
\starttext
\startbodymatter
  \setuppagenumbering[conversion=numbers]
  \setcounter[userpage][1]

  \startchapter[
    title={Chapter},
    reference=sec:chapter,
  ]

  \startsection[
    title={Section},
    reference=sec:chapter,
  ]

  \placefigure[]{}{%
    \externalfigure[sheep.jpg][
        width=\textwidth,
      ]
  }

  \startsubsection[
    title={Equipment},
    reference=sec:equipment,]
    \input knuth
  \stopsubsection

  \stopsection
  \stopchapter\stopbodymatter
\stoptext

Actual results: the image appears as "undefined".
Expected results: the image appears, stretched to fill the column.
Work around 1: use wfactor.
Work around 2: \defineexternalfigure[fullwidth][factor=fit] and then use
\externalfigure[file.jpg][fullwidth]

Kind regards.